The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others, that use micro-lenses. Semiconductor-based displays using micro-lenses are also known.
It is desirable to maximize the amount of light received by the photo-conversion devices of an imager. One way to increase the light received by the photo-conversion devices is to increase the amount of light received by micro-lenses, which collect external light and focus it on the photo-conversion device. Another way is to enhance the positioning of the focal point of each micro-lens to ensure that much of the light received by the micro-lenses is focused on the photo-conversion devices.
In conventional micro-lens fabrication, photo-resist is reflowed to form micro-lenses having radii proportional to the pixel sizes. It is hard to form a small micro-lens while maintaining the appropriate focal length in conventional micro-lens fabrication. The smaller a micro-lens made through conventional processes is, the shorter the focal length becomes. Thus, it would be advantageous to form micro-lenses that are aspherical and have differently sized radii.
Micro-lenses may be formed through an additive process in which a lens material is formed on a substrate and subsequently formed into a shaped micro-lens. Micro-lenses also may be formed by a subtractive process. Known subtractive processes, however, are complex and manufacturing micro-lenses from such known processes is difficult.